1. Field of the Invention
This invention relates to a light control device, and more particularly to a light control device such as a shutter or an automatic diaphragm which mechanically fully closes or reduces an opening through which light passes, by means of a plurality of light-intercepting blades. The invention is especially concerned with the structure of each of the light-intercepting blades forming such a light control device.
2. Description of the Prior Art
A light-intercepting device used in a mechanical light control device of the prior art, such as a shutter or an automatic diaphragm, is constituted by a plurality of blades formed of metal plates or plastic plates. By providing high-speed movement of this light-intercepting device, a short exposure time can be obtained and the accuracy of the exposure time can be improved in still cameras. Reducing the drive force for the shutter or the automatic diaphragm is advantageous that the wind-up of the camera is lighter and the durability of the shutter or the diaphragm is improved. To reduce the drive force and moreover move the light-intercepting device at a high speed, it is necessary to reduce the mass of the blades. Blades having one or both surfaces thereof subjected to so-called thinning, which reduces the thickness of the blades, other than the marginal edge portion, have been proposed in U.S. Pat. No. 4,401,380, granted Aug. 30, 1983, and assigned to the assignee of the present invention.
However, the light-intercepting device usually has its partly reducing formed of thin plates, and making the thickness thereof unavoidably results in decreased strength of the blades. Particularly, at the termination of the movement of shutter blades, a brake is applied in a very short range and therefore, the impact applied to the blades themselves is very great. In the case of a square type focal plane shutter in which the forward blade group and the rearward blade group each comprise several blades, a brake is after applied not by directly stopping the blades but by stopping the blade driving member (blade arm). The impact applied on the blades is greatest in the root of the blades (the neighborhood of the junction portion between the blade arm and the blades) where the inertia of the entire blades acts, and flexure or bending may be caused in that portion.
Partly reducing the thickness of the shutter blades makes the blades lighter in weight, but is disadvantages as mentioned above and, simply making the blades lighter in weight does not always lead to higher speed of movement of the blades.